[Wavytone]

You have a lot to learn, grasshopper. As Confucius say: Those who bite off more than they can chew are destined to choke.

Try grinding, polishing and figuring an 8" f/5 mirror first to understand what is involved before attempting anything larger.
Maybe then a 13-14" thin mirror at f/4.5 if you are confident and (a) the 8" was well polished and (b) has a good figure.
Only then try a 20".

Once you understand what is involved I suggest you will quickly realise why no-one makes mirrors by hand anymore, that died out 20 years ago when the Chinese entered the market. There is a good reason why you see unused large blanks coming up for sale on places like Cloudynights.

FWIW I have made a 6" f/8, 4" f/3.5, 8" f/6 and a Ramsden eyepiece, long ago.